In tool equipment involving moving parts, it is desirable that the motion quickly stop after the electric power is turned off to avoid possible injury to the user. In those tools driven by fractional Horsepower induction motors, the rotor of the motor is relatively heavy. Moreover, the coasting motor is very quiet as compared to a fractional Horsepower universal-type electric motor having a commutator and brushes, so that the user is not audibly alerted to the continuing coasting motion of the induction motor and its associated driven tool. This quiet coasting can occur in induction-motor-driven food processors.
Such food processors are kitchen appliances utilizing a variety of interchangeable rotary tools, as for example, knives, blades, cutting discs, and rasping discs for performing such operations as cutting, slicing, mixing, blending, grating, shredding, chopping and pureeing, etc.
Known food processors generally include a supporting base structure containing an induction motor, a work bowl adapted to be seated on the base with tool drive means in the work bowl. The specific rotary tool needed for a desired food processing application is removably engaged with the tool drive. When the motor is actuated, such a tool spins rapidly within the work bowl. A cover having a feed tube is removably mounted to the top of the work bowl. Food to be processed is inserted through the feed tube in the cover and into the work bowl to be processed by the rotary tool.
Fractional Horsepower induction motors of the type generally referred to above are used in many such food processors. Such induction motors are energized by single-phase electric power, and so they include a main winding and an auxiliary starter winding. There is a phase-shifting electric component connected in series with the auxiliary winding, such component usually being a capacitor, and the starting winding is energized briefly to start rotation of the rotor. The specific tools used by the appliance are rotated at relatively high speeds during operation to perform their required functions.
In known food processors, the induction-motor-driven rotary tool can revolve at speeds of approximately 1,800 RPM. Thus, it is important that the processor include a braking system which will minimize coasting of the rotary tool after the motor has been de-energized. Such braking systems provide the user with quick access to the processed food in the work bowl after the motor has been turned off and also minimize the possibility of injury to a user if the bowl cover is removed quickly and a hand is immediately inserted into the bowl.
Home appliances such as food processors are often equipped with protective switches which automatically shut off the power when a cover is removed from the zone where the tool is located.
In known braking systems for induction motors, as for example the one illustrated in U.S. Pat. No. 4,241,302--Benjamin, which is assigned to the same assignee as the present application, a starting capacitor of the induction motor is electrically coupled to the windings of the induction motor to produce an electromagnetic braking effect within the motor when a control switch is in the "off" position, the braking effect is produced by converting the rotational mechanical energy into electrical energy which is quickly dissipated as heat.
In practice, it has been found that the braking effect afforded by the starting capacitor of the induction motor is not so great as desired. To enhance the braking effect, this Benjamin patent discloses the use of a second capacitor, separate from the starting capacitor, and having a capacitance from 2 to 10 times larger than that of the starting capacitor, said second capacitor becoming connected to the windings of the induction motor to produce an electromagnetic braking effect after the induction motor has been turned off, using resonance between the capacitors and the windings for optimally converting the rotational mechanical energy into electrical energy which is dissipated as heat for suddenly stopping the motor.
The starting capacitor normally has a capacitance value of a size to cause the current through the starting winding to be approximately 90.degree. out of phase with the current through the running winding(s) for producing a strong starting torque.
In order to provide a greatly enhanced braking action, this second capacitor is chosen to have a capacitance value considerably larger than the starting capacitor, being chosen to be resonant with the running winding, or with both running windings in parallel if a second running winding is present, at a frequenty in the range from approximately 15 Hz to 40 Hz for producing quick-acting powerful braking action.